The present invention relates to a sintered carbonitride alloy with titanium as the main component, so-called cermets, intended for milling, drilling and turning of metal, which alloy has a very good toughness behavior in combination with good wear resistance.
Classic titanium-based cutting tool material was based on titanium carbide, molybdenum carbide and nickel. These materials were used for high speed finishing owing to their extraordinary wear resistance at high cutting temperatures. The toughness behavior and resistance against plastic deformation were not satisfactory, however, and so the area of application was rather limited.
Development has proceeded and the range of application for sintered titanium carbonitride based alloys has been considerably enlarged. The toughness behavior and the resistance against plastic deformation for these alloys have been considerably improved.
An important development of titanium-based hard alloys is the substitution of carbon by nitrogen in the hard constituents. This decreases the grain size, usually 1-2 .mu.m, of the hard constituent in the alloy which leads to the possibility of increasing the toughness behavior.
In general, nitrides are more chemically stable than carbides which results in lower tendencies to sticking of workpiece material or wear by dissolution of the tool (so-called diffusional wear).
For the binder phase, the metals of the iron group are used, often Co and Ni in combination. The amount of binder phase is generally 5-25% by weight. Besides titanium, the other metals of the group IVA, VA, VIA are normally used as hard phase formers such as carbides, nitrides and/or carbonitrides. There are also other metals used, for example, Al, which sometimes are said to harden the binder phase and sometimes improve the wetting behavior between hard phase and binder phase.
A very common or even normal microstructure of sintered carbonitride alloy consists of a core-rim structure. For example, U.S. Pat. No. 3,971,656 discloses a sintered carbonitride alloy which comprises Ti- and N-rich cores and rims rich in Mo, W and C. From Swedish patent application no. 8902306-3, it is known that different combinations of duplex core-rim structures in well balanced proportions give improved wear resistance or toughness behavior properties. The distribution of hard constituent particles containing titanium, tantalum and tungsten especially affects the cutting properties for different sintered titanium-based carbonitride alloys with the same overall chemical composition. The difference in cutting behavior remains even when the overall carbon content varies.
From the literature on titanium-based carbonitride alloys, it is apparent that the trend of substituting carbon by nitrogen is very common. It has been shown that properties related to toughness behavior in metal cutting operations (turning, milling and drilling) in general have been improved by substituting titanium carbide by titanium nitride or titanium carbonitride. This holds for a nitrogen content up to a certain level where the wetting properties no longer permit a sintered material without pores. Although diffusional wear (crater wear) resistance is improved with increasing nitrogen content, wear resistance in general decreases with increasing nitrogen content.
The microstructure and the metal cutting properties of sintered titanium-based carbonitrides with the same overall chemical composition vary. For a production process similar to the process generally used in the production of cemented carbides, including pressing and vacuum sintering, different hard constituents behave differently during the liquid phase sintering. Some of the hard constituent particles remain as cores in the sintered carbonitride alloy and inherent more or less completely their metallic composition, while others are completely dissolved and affect the rim structure formation.
U.S. Pat. No. 4,935,057 discloses a method of making a titanium-based carbonitride alloy characterized by the steps of preparing a first powder for forming the core, preparing second powders for forming the rims and preparing a third powder for forming the binder phase. Said powders are milled, compacted and sintered. The first powder is formed of at least one compound selected from the group consisting of TiC, TiCN, (Ti,Ta)C and (Ti,Ta)(C,N).